Simplify the module to just use cb(null, file.clone())

See this issue https://github.com/ben-eb/gulp-rsvg/pull/2

Tested this out and it works pretty well. It lets you compose gulp tasks like so:

```js
var gulp   = require('gulp');
    clone  = require('./');
    concat = require('gulp-concat');
    es     = require('event-stream');
    uglify = require('gulp-uglify');

gulp.task('scripts', function() {
    var stream = gulp.src('*.js').pipe(concat('main.js'));

    var merged = stream
        .pipe(clone())
        .pipe(uglify())
        .pipe(gulp.dest('prod'));

    return es.merge(stream.pipe(gulp.dest('dev')), merged);
});
```

This would allow you to create two JS bundles; one concatenated and one concatenated and minified. :smiley: